1. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium which is excellent in thermal stability of recorded magnetization and suitable for a high density magnetic recording and a magnetic storage apparatus using the same.
2. Descriptions of the Related Arts
In an longitudinal magnetic recording system used in magnetic disc apparatuses of nowadays, to increase a linear recording density, demagnetization fields in recorded bits must be reduced by reducing a product (Brxc2x7t) of remanent magnetization (Br) of a magnetic film as a recording medium and a thickness (t) of the magnetic film. At the same time, a coercive force of the magnetic film must be increased. Moreover, to reduce medium noise, an easy axis of magnetization of the magnetic film must be made to be in parallel with a substrate surface, and a reduction in both an average crystal grain size and the crystal grain size distribution is necessary. To satisfy such an object, a thickness of the magnetic film must be made as thin as 20 nm or less, and the crystal grain size must be made as minute as 10 nm. However, in the medium having such magnetic crystal grains micronized, there is a problem that the recorded magnetization is reduced due to thermal agitation, and the thermal decay of the recorded magnetization is obstacles to realize a high density recording.
On the other hand, a perpendicular magnetic recording system is the one which forms recorded bits so that a magnetization direction of a recording medium is perpendicular to a medium surface and magnetization directions in recorded bits adjacent to each other are in anti-parallel. Since demagnetization fields at magnetization transition regions are smaller compared to the longitudinal recording system, the medium noise can be reduced, and recorded magnetization at high recording densities can be stably retained. Also in the perpendicular magnetic recording, to increase the linear recording density, it is necessary to reduce medium noise generated from irregular magnetic domains which are formed inside the recorded bits and the magnetization transition regions. To reduce the medium noise, an easy axis of magnetization of the magnetic film must be oriented so as to be perpendicular to the substrate surface. At the same time, a dispersion angle of the easy axis of magnetization is made small, and a crystal grain size of the magnetic layer must be controlled.
As a perpendicular magnetic film, a Co alloy having a hexagonal closed-packed (hcp) structure is mainly used. When a CoCr-based alloy thin film which is obtained by adding Ta, Pt, Rh, Pd, Ti, Ni, Nb, Hf and the like to Co and Cr as a main component is used, good properties can be achieved as the perpendicular magnetic film. The Co alloy thin film has an easy axis of magnetization in a c-axis direction of the hcp structure, that is, in a  less than 00.1 greater than  direction, and the easy axis of magnetization is oriented in a direction perpendicular to a film surface thereof. The magnetic thin film is formed by use of a vacuum evaporation method and a sputtering method. A c-axis orientation of the Co alloy thin film to a direction perpendicular to a film surface thereof must be enhanced, and a crystal grain size must be controlled to a suitable size. To achieve such an object, reform measures in which an underlayer for structure control is formed between a substrate and a magnetic film have heretofore been adopted.
On the other hand, as a perpendicular magnetic recording medium which is made of materials other than the CoCr-based alloy series materials, amorphous alloy made of rare earth-transition metals such as TbFeCo has been known. Furthermore, also a thin film formed of a multilayer film such as (Co/Pd)n and (Co/Pt)n has been investigated, which is obtained by alternately laminating Co films of a thickness of 1 nm or less and Pd films of a thickness of 1 nm or less or by alternately laminating the Co films and Pt films of a thickness of 1 nm or less. These films show a strong exchange interaction between magnetic grains unlike the CoCr-based alloy film, and a squareness thereof is approximately 1. Therefore, these films are excellent in thermal stability and a signal-to-noise (S/N) ratio at low recording densities.
In Japanese Patent Application Laid-Open No. 9(1997)-91660, a magnetic recording medium in which first and second recording layers having different properties from each other are laminated is disclosed. In Japanese Patent Application Laid-Open No. 12(2000)-113442, a magnetic recording medium is disclosed, in which a magnetic film having a high magnetic anisotropy energy is formed on/beneath a magnetic film made of a Co alloy as a main recording layer, thus reducing medium noise and thermal fluctuations.
Though the foregoing perpendicular magnetic recording medium made of the CoCr-based alloy shows a low noise, this medium does not have sufficient resistance to the thermal fluctuations. On the other hand, though the perpendicular magnetic recording medium made of the amorphous alloy of the rare earth-transition metals such as TbFeCo and made of the multilayer film such as (Co/Pd)n and (Co/Pt)n is excellent in the thermal stability and the signal-to-noise (S/N) ratio at low recording densities, the perpendicular magnetic recording medium has a problem that the medium noise at high recording densities is large. The perpendicular magnetic recording films as the prior arts disclosed in Japanese Patent Application Laid-Open No. 9(1997)-91660 and 12(2000)-113442 use the multilayer structure film such as Co/Pd and the amorphous alloy film containing the rare earth-transition metals as the second magnetic film. The former has a problem that there is a difficulty in manufacturing it industrially, and the later shows a poor corrosion resistance because the latter contains the corrosive rare earth metals. Moreover, the inventors of the present invention found newly that when a rare earth-3d transition metal amorphous alloy is used as the recording layer, this layer and carbon used as a protective film react with each other, so that sufficient thermal stability and signal-to noise ratio cannot be obtained at a thin area in which a thickness of the recording layer is equal to several nm or less.
With a recognition of the foregoing problems, an object of the present invention is to provide a perpendicular magnetic recording medium and a magnetic storage apparatus which are capable of solving the foregoing drawbacks of the prior arts, excellent in thermal stability and an S/N ratio, and suitable for a high density magnetic recording.
To achieve the foregoing object, the perpendicular magnetic recording medium of the present invention comprises: a magnetic layer formed above a substrate, which contains Co and Cr as main components; a first layer formed on an opposite side of the magnetic layer relative to the substrate, the first layer including an amorphous alloy layer containing rare earth metals and 3d transition metals as a main component; and a second layer formed on the first layer, the second layer containing Co and Cr. The second layer may be one formed of Co and Cr.
The first layer may be a multilayer film composed of an amorphous alloy layers and other layers, the amorphous alloy layers containing rare earth metals and 3d transition metals as a main component. As layers inserted between the amorphous alloy layers, alloy films containing Co and Cr as a main component may be used. By forming the first layer to be multilayered, it is possible to control a decay rate of signals to a negligible level for practical use, and to increase an S/N ratio.
A thickness of the first layer should range from 2 to 10 nm. By setting the thickness of the first layer to the range from 2 to 10 nm, it is possible to control the decay rate of the signals to the negligible level for practical use, and to obtain a high S/N ratio. Satisfactory thermal stability cannot be obtained at a region where the thickness of the first layer is less than 2 nm. When the thickness thereof is equal to 10 nm or more, the S/N ratio is lowered though the thermal stability is not a problem.
The first layer can contain TbFeCo, TbCo or TbFe as a main component. The first layer is suitable for industrial productions by allowing the first layer to contain these materials as the main component.
A thickness of the second layer should range from 0.5 to 10 nm. By setting the thickness of the second layer to the range from 0.5 to 10 nm, it is possible to obtain good magnetic properties and recording characteristics at a region where the thickness of the first layer is several nm or less. When the thickness thereof is less than 0.5 nm, a reaction of the second layer with C (carbon) of a protective film cannot be suppressed sufficiently, and thermal stability and an S/N ratio are lowered. When the thickness of the second layer is greater than 10 nm, the thermal stability is improved compared to a case where there is no second layer. However, lowering of a recording resolution and the S/N ratio is caused because a magnetic spacing, which is defined as spacing between a head and a soft-magnetic underlayer, is widened, and because magnetic properties such as coercive force and saturated magnetization of the second layer are not excellent.
The magnetic storage apparatus of the present invention comprises a perpendicular magnetic recording medium and a thin film head including a magnetic recording head and a signal reproduction head, wherein the perpendicular magnetic recording medium has: a magnetic layer containing Co and Cr as a main component, formed above a substrate; a first layer including an amorphous alloy layer formed on an opposite side of the magnetic layer relative to the substrate, the amorphous alloy layer containing rare-earth metals and 3d transition metals; and a second layer containing Co and Cr, formed on the first layer including the amorphous alloy layer. As the magnetic recording head, a ring-type head or a single-pole-type head can be used. As the signal reproduction head, a magnetoresistive head, a spin valve type head or a tunneling magnetoresistive head can be used.